This is a known problem with AutoREALM. The way to adjust the grid size is with the slider bar on the graph paper toolbar, which shows you how many units/inch on the page. However, because of an existing bug it is not always accurate (as you may have already discovered), and is really only an estimation. If you start with a new map, change the scale as you desire and print a test page if you've got the right scale or not. If not, you can adjust it from there, and once you do find the right settings, save it!
The use of inches vs. centimeters is based on your Windows international settings. To change it, go into the Control panel and change your setting from metric to English under (depending on your version of Windows) either International Settings, Local settings, or Regional Options.